The aromaticity of cyanine dyes: ring current and valence bond calculations

Cyanine dyes are interesting molecules due to their optical properties, in particular their absorption in the near infrared window. Odd/even effects for these dyes were observed [N. P. Neme, T. L. C. Jansen and R. W. A. Havenith, Phys. Chem. Chem. Phys., 2024, 26, 6235-6241], due to interaction with the polymethine chain. Here, we present a study of the aromatic properties of oxidized cyanine dyes 1(n) using the ipsocentric method to calculate induced ring currents and using valence bond theory. The even members of this series show an induced diatropic ring current, while the odd members show an indued paratropic ring current. Hence, for n even, the ring is aromatic. The aromatic behaviour of the five-membered ring aligns with the possibility to draw ionic resonance structures in the five-membered ring that allows the delocalization of a negative charge within the ring, leading to a six π electron ring. For n odd, the induced ring current is paratropic, while a positive charge delocalizes in the five membered ring (a 4π electron circuit), hence, its aromatic behaviour. This steering of aromatic/antiaromatic properties can have a profound effect on the reactivity of the molecules, and impact their photophysical properties. This may be an extra handle for the design of organic molecules for electronic devices.